This invention relates generally to refrigeration systems and in particular to a refrigeration apparatus having an electrically controlled refrigerant storage device to adjust the mass of the refrigerant circulating in the operating loop of the system so as to optimize the operation of the refrigeration apparatus.
Conventional refrigeration systems having moderate capacity (e.g., less than 5 tons) typically include a compressor, a condenser, an evaporator, and a fixed expansion device, such as an orifice or capillary tube. The expansion device is used to introduce a pressure drop in the refrigerant as it passes from the condenser to the evaporator. It is common in refrigeration systems used in refrigerators and small heat pump systems (which typically have a capacity of less than one ton) that the compressor speed and volume are fixed (that is not variable) and the refrigerant charge (that is, the mass of refrigerant circulating through the operating loop of the system) is also fixed. As a consequence, such a system's refrigerant charge can be tuned for most energy-efficient operation for only one set of nominal operating conditions (that is, to meet a given cooling demand); further, in other than that one set of nominal operating conditions the refrigeration apparatus continues to operate but is detuned (that is, one or more components of the system no longer operate at optimal efficiency in conjunction with the other components in the system. One example of a component of the refrigeration apparatus having on optimal point of energy efficient operation is the compressor motor, which typically has one design load that provides the best electrical efficiency for the motor.
The variation in cooling demands placed on the refrigeration apparatus that can effect the optimal operation of system components include changes in ambient conditions and changes in the refrigerator compartment being cooled (e.g., freezer versus fresh food). For example, in conditions of high ambient temperature and humidity coupled with the demand to cool the freezer compartment, the refrigeration apparatus will see a large refrigerant differential temperature between the evaporator and the condenser. Under these conditions the compressor motor can pump refrigerant through the system at a particular mass flow rate. Under less adverse conditions, such as cooler ambient conditions, the compressor motor is pumping refrigerant across a smaller pressure differential (because the temperature differential of the refrigerant through the system is also less) and thus is capable of pumping refrigerant at a higher mass flow rate. In most conventional systems, however, the refrigerant mass is fixed, and thus in operating conditions other than the design (or nominal) conditions, energy will be wasted as the compressor motor will operate at a less efficient point on its efficiency curve.
It is desirable to improve the energy-efficiency of refrigeration systems by enabling them to meet a range of cooling demands and controlling the system to respond to the current cooling demands while operating at near to optimal compressor loads as possible. It is also desirable that an energy saving system be readily fabricated and easily adapted to the refrigeration systems presently manufactured such that the cost of acquiring and operating the system does not exceed the economic benefits of the improved energy efficiency.
It is thus an object of this invention to provide a refrigeration system that improves the energy efficiency of the system through selectively controlling refrigerant mass being used for cooling purposes in the system by use of a controllable and variable refrigerant storage device so as to provide a refrigerant charge that will load the compressor to its optimal energy efficiency in a variety of normal operating environmental conditions and operator set points. Further, such a system for varying refrigerant charge enhances manufacturing flexibility by providing refrigeration apparatus that can readily accomodate normal manufacturing tolerances for components of the refrigeration system (and thus the effect of such variations on the operation of the whole system).